1. Field of the Invention
The present invention relates to a magnetic recording system comprising perpendicular magnetic recording media.
2. Description of the Related Art
The magnetic recording system is such that the head is moved radially of a rotating disk and accurately set in position on an intended data track to write and read information magnetically. A top plane view of the interior of a housing (enclosure) of a typical magnetic recording system is shown in FIG. 10, and a sectional view of the magnetic recording system is shown in FIG. 11. These diagrams illustrate a magnetic recording system configured of six heads, three disks, a rotary actuator, a voice coil motor, a head amplifier and a package board.
The three disks are fixed on a single rotary shaft and driven at the rotation speed of 3000 to 15000 rpm around a point A by a spindle motor. The six heads are fixed on a single comb-shaped arm and rotationally driven around a point B by a rotary actuator. This mechanism permits the heads to move freely radially of the disk. The rotary actuator is suitable for reducing the size of the mechanism, and therefore is employed by all the magnetic recording systems recently made available on the market. Also, for detecting the radial position of the heads, servo areas are formed at substantially regular intervals of rotation angle on the disk. A detailed explanation will be given later of the arrangement of the servo areas and the data areas and means for detecting the radial position of the heads based on the servo areas. A package board has mounted thereon a hard disk controller (HDC), an interface circuit, a signal processor, etc. The head amplifier is often mounted in the enclosure in the neighborhood of the heads in order to improve the SN ratio and the transfer rate.
A plane view with the disk partially enlarged is shown in FIG. 12. The head can be moved to an arbitrary radial position on the disk by the rotary actuator, but when writing or reading data, is fixed at a specific radial position. As shown in FIG. 12, concentric tracks are formed substantially equidistantly. By way of explanation, only five tracks are shown in solid lines. Actually, however, the tracks, which are magnetically formed, cannot be optically viewed directly. Although the track width is shown in enlarged form, an actual magnetic recording system has tracks in the number of several tens of thousands formed at intervals narrower than 1 .mu.m from the inner to outer peripheries of the disk.
For performing the operation of following a specific track, a technique is widely used in which a special pattern called the servo pattern is recorded before product shipment and a head position signal is obtained from this pattern. This technique is disclosed in JP-A-58-222468. The servo pattern is formed in the portions designated as servo areas in FIGS. 10 and 13. Each servo area and each data area are spaced from each other through a gap for absorbing the variations of the rotation speed. Further, each data area is divided into sector blocks of about 600 bytes including the user data of 512 bytes and management information. The main difference between the servo data and the data area is that the data area is rewritten frequently in response to a user instruction whereas the servo area is not rewritten after product shipment.
About 50 to 100 servo areas are formed at substantially equal angular intervals on the disk. The number of the data areas is greater than that of the servo areas, and therefore several data areas exist between given two servo areas. FIG. 13 shows an example in which a data area #1 is arranged between servo areas #1 and #2, and the data area #1has three sector blocks #1 to #3 for each of four tracks #1 to #4. An actual magnetic recording system has ten thousand or more tracks, and a portion of the tracks is shown in perpendicularly enlarged form in FIG. 13.
A pattern having a bit-direction timing synchronized between radially adjacent tracks is recorded in each servo area. For forming such a special pattern, a clock synchronized with disk rotation is required. A servo pattern is normally formed with a device called the servo track writer having the aforementioned function. A method of forming servo areas in this way is disclosed, for example, in JP-A-64-48276.
A general structure of a pattern formed in the servo areas and a method of producing the head position signal called the servo information from the servo pattern are shown in FIG. 14. In the pattern shown in FIG. 14, an ISG (Initial Signal Gain) area is a continuous pattern formed to reduce the effect of variations in the flying height and the magnetic characteristics of the recording film of each disk. A servo decoder reproduces the ISG area by turning on an automatic gain control (AGC). At the time point when a SVAM (SerVo Address Mark) area is detected, the AGC is turned off thereby to realize the function of standardizing the reproduction amplitude of the subsequent burst areas using the amplitude of the ISG area. A gray code area is for describing the track number information of each track by the gray code. The sector number information may also be described in this area. A burst area has a hound's tooth check pattern for producing accurate radial position information and is required for the head to follow each track accurately. This pattern is configured of a combination of A and B bursts formed between the center lines of adjacent servo tracks over the boundary of the particular tracks on the one hand and a combination of C and D bursts formed about the center lines of the servo tracks on the other hand. A pad area is a pattern formed for absorbing the delay of the servo decoder and peripheral circuits so that the servo decoder can maintain the clock generation during the reproduction of the servo area.
The head reproduces the servo area while running along the position curve C indicated by arrow from left to right in FIG. 14. A part of the waveform reproduced by this operation is shown in FIG. 15. For facilitating the understanding, the reproduced waveforms of the SVAM area, the gray code area and the pad area are not shown. The servo decoder detects the amplitude of the four burst areas A to D. The reproduction signal of each burst area is converted into a digital value by an A/D converter, and the amplitude is detected by integration or Fourier computation. The amplitude difference between the A burst area and the B burst area makes up an N position signal. FIG. 15 includes an equation for standardizing the amplitude difference with the ISG amplitude. This function is implemented by the servo decoder controlling the AGC in such a manner as to secure a constant amplitude of the ISG area. In similar fashion, the amplitude difference between the C burst area and the D burst area constitutes a Q position signal. The head position signals produced in the manner described above are shown in FIG. 16. The N position signal assumes 0 at position B where the head center covers the A burst area and the B burst area equally, and changes between positive and negative values substantially in proportion to the amount of displacement from the center position. From the reproduced waveform (reproduced waveform at position C in FIG. 14) shown in FIG. 15, for example, the N position signal for position C in FIG. 16 can be obtained.
A controller for controlling the position of the magnetic head produces a continuous position signal by comparing the absolute values of the N and Q position signals and connecting them by reversing the positive and negative values. In many servo patterns, a voice coil motor is controlled by setting the position associated with the N position signal of 0 as a target of following. Based on the difference between the position signal and the target position, the optimum current value charged to the voice coil motor is calculated. Then, predetermined operations such as the following and seek operations are performed.
The steps of forming the burst area will be explained briefly with reference to FIGS. 17(a) to 17(e) and 18(f) to 18(i). The portions defined by thick lines are patterns recorded in each step, and the width along the transverse direction of the pattern corresponds to the width of the recording track. A recording current pattern corresponding to each recorded pattern is shown under the portion defined by each thick line. As shown in FIGS. 14 and 15, the heads are moved at intervals of the data track, i.e. one half of the track pitch, while recording different patterns in phase. Some portions are written additionally, while other portions are erased by DC field. As a result, the burst area in the shape of hound's tooth check is recorded.
FIGS. 19(a) to 19(d) show the relation between a recorded magnetization pattern and a reproduced wave for both the longitudinal magnetic recording system and the perpendicular magnetic recording system in comparison. The longitudinal magnetic recording system has no response to the DC magnetization, and a single-peaked output is produced only at the transition. The reproduction of the recorded magnetization pattern shown in FIG. 19(a) assumes a waveform as shown in FIG. 19(b). As a result, a reproduced waveform as shown in FIG. 20(a) is obtained for the servo pattern described above, and the integrated signal of absolute value of waveform for producing the position signal information assumes a form as shown in FIG. 20(b). This indicates that the amplitude of each burst and that of a corresponding integrated signal coincide well in magnitude with each other.
However, a double-layer perpendicular recording medium having a soft magnetic under layer has a response to DC magnetization, and the reproduction for the recorded magnetization shown in FIG. 19(c) assumes a reproduced waveform as shown in FIG. 19(d). Thus, the reproduction for a servo pattern similar to that for longitudinal recording assumes a waveform which undesirably has a DC offset as shown in FIG. 21(a). The integrated signal of absolute value of waveform for producing the position signal information assumes a form as shown in FIG. 21(b), which fails to correctly represent the amplitude level of the burst signal. The actual circuits of the reproduction system such as the AGC and the read amplifier have a characteristic of lowering the DC component. Therefore, the reproduced waveform is distorted as shown in FIG. 22, and when integrated, a correct position signal cannot be produced due to the effect of the DC offset.
The burst signal portion of the burst area is arranged as described above in such a manner as to be surrounded by a large DC erased area as shown in FIG. 2. The feature of the double-layer perpendicular recording is that the shorter the wavelength, the smaller the demagnetization field in the recording bits, and hence the longer the wavelength, the larger the thermal demagnetization. FIG. 23 shows an example of the simulation result of the secular variations of the reproduced output for the recording densities of 20 KFCI, 100 KFCI and 300 KFCI, respectively. It is seen that the lower the recording density, i.e. the longer the wavelength for bits, the larger the output reduction. Due to a similar effect, the magnetic field generated by magnetization of the DC erased area described above affects the adjacent servo signal area and promotes the thermal demagnetization of the servo signal area.
In the case where the DC magnetization is recorded as a base for recording bits, a phenomenon is reported in which the track edge of the recorded bits shifts along the track width according to whether the particular track end coincides with the polarity of the base DC magnetization. The end of the burst signal area also shifts due to a similar phenomenon, thereby deteriorating the position signal quality.
Another problem of the prior art is that the maximum bit length of the signal recorded in the servo area is longer than the maximum bit length of the data area, and under this condition, the anti-signal decay performance of the servo area is weakest from design aspect, thereby making it impossible to secure reliability.